The invention relates to a method of supporting a plastic container exposed to the combination of internal pressure variations and elevated temperatures during product filling and packaging thereby eliminating distortion induced by the heat and internal pressure.
Plastic containers for packaging foods have been widely accepted in some applications, such as soft drinks, bottled water and juices, due to their well known advantages over conventional glass and metal containers. Substantial reductions in weight and the low cost of using plastic containers creates circumstances where it is highly desirable to expand the application of plastic containers if possible to other areas in packaging. However, in many cases, conventional use of plastic containers results in problems when the plastic containers are exposed to an elevated temperature which reduces the plastic strength, internal vacuum which collapses a container, or high internal pressures combined with heat which tends to bulge or distort the plastic walls of the container to an acceptable degree. To date plastic containers have not replaced conventional use of glass bottles or jars for packaging many food products, which are pasteurized or retort processed after filling the container.
Thermal distortion of the container may cause unacceptable bulging of the side walls, bottom surface distortion can cause the container to lean to one side, distortion of the bottle neck area can create problems in sealing the containers after hot filling, expansion of the side walls can cause difficulty in attaching labels and any distortion of the container detracts from the aesthetic appeal of the packaging itself.
There have been attempts to modify blow moulded polyester containers or PET bottles in order to enable hot filling or retort processing with limited success. For example, heat setting of the plastic container or co-extrusion of heat resistant materials with other less costly resins have been applied however at significantly increased cost and increased manufacturing cycle time. In heat treating, the thermoplastic material is subjected to heat wherein the crystal structure is changed to increase the heat resistance to heat distortion of the final package.
In general however, heat setting and addition of heat resistant materials involve unacceptable increases in costs that detract from the principle advantages of using plastic containers.
In the case of hot filling of plastic containers, the conventional manner of dealing with a resultant negative or vacuum pressure within the plastic container is to use specially designed vacuum panels in the lateral sides of the bottle or in the bottom surface which bow inwardly to deform and accommodate the product shrinkage and negative internal pressure. Large collapsible panels in the sidewalls of PET containers severely restrict the design of the package itself and limit the application of labels. Also the expense of specially designed dies, maintenance of separate bottle inventory for hot fill applications and moulding machine change over costs result from using a different bottle design for different product processing methods.
As is known in the art, hot fill applications involve heating of comestible products to a temperature approximately 140xc2x0 F. to 205xc2x0 F. (60xc2x0 C. to 96xc2x0 C.), placing the hot product in the container and sealing the container. During cooling of the product however, the hot product and hot gas in the head space shrink in volume. Cooling after sealing therefore creates a negative internal pressure or vacuum within the final filled container. Further shrinkage of the product occurs if the package is refrigerated below ambient temperature for storage. Without collapsible vacuum panels in the side of the plastic container, the resulting pressure differential creates a net external pressure causing the container to buckle or collapse inwardly, sometimes referred to as xe2x80x9cpanelingxe2x80x9d.
Therefore, conventional methods of adapting plastic containers to products which are heated during processing and packaging have met with limited success. Disadvantages include the risk of heat distortion which can be addressed by specially designed vacuum collapsing panels or relatively expensive heat set and heat resistant containers.
In the case of hot filled aluminium cans, it is well-known that negative internal pressure caused by hot filling can be counteracted by adding liquefied nitrogen gas or dry ice immediately before sealing the aluminium container. During this process the nitrogen or carbon dioxide gas created on contact with the hot product creates a positive pressure within the sealed aluminium container. The relatively high strength aluminium container can resist a high internal pressure during processing. When the product cools, the shrinkage of the product and negative pressure resulting is countered by a greater positive pressure created by the gas within the sealed container to produce a residual net positive pressure in the final container. Examples of this prior art process are described in U.S. Pat. No. 4,703,609 to Yoshida et al. and U.S. Pat. No. 4,662,154 to Hayward.
Attempts have been made to apply the same technology to plastic containers with limited success. U.S. Pat. No. 5,251,424 to Zenger et al. describes essentially the same process applied to hot filling of a plastic PET container. In Zenger, the hot filled product is poured into a plastic bottle, liquid nitrogen is dosed into the hot product immediately before closing the container and the container is permitted to cool to storage temperature. Normally, a hot filled product will shrink and create a vacuum within the plastic container which conventionally has been addressed with vacuum panels. However, Zenger et al. describes a method of increasing internal positive pressure through use of liquid nitrogen gas which counteracts the negative pressure created on cooling of the hot filled product.
In theory, the Zenger et al. method permits a conventional PET plastic container to be used for hot filling applications, which results in cost savings. However, in practice it has proved extremely difficult to implement. The accurate dosing of liquid nitrogen or solid dry ice to the precision required for use of plastic containers has proved illusive.
As is apparent to those skilled in the art, by nature a heat formable blow-moulded plastic bottle is very sensitive to variations in the heat absorbed by the material. The uniformity of plastic container composition and the uniformity of heat of the hot filled product are such that it is extremely difficult to predict with sufficient accuracy the performance of the hot filled plastic container. Variations in product density, heat distribution, and physical forces applied to the product filled plastic container during handling and packaging operations can have significant effect on the performance when dosed with liquid nitrogen to increase the internal pressure.
Further, the accurate dosing of liquid nitrogen gas or solid dry ice into the product prior to capping is extremely difficult to accomplish with the required accuracy. In the case of liquid nitrogen, the size of a liquid drop can vary significantly and the volume of liquid nitrogen required is in the order of one or two drops only. The inherent inaccuracy is not a particular difficulty when the packaging has a high margin of safety in its strength such as for example in the dosing of product packed in aluminium cans.
In the case of PET plastic containers however the packaging when heated is at a significantly reduced structural strength due to the heat sensitivity of plastic materials. As well, the dosed product when capped subjects the packaging to the most extreme internal pressure that it will experience in its service life. When the gas forms to create a high internal pressure, the packaging is heated and has a reduced strength, the container is also subjected to hydrostatic forces from the liquid product within the container and is usually in transit on conveyors or otherwise subjected to external physical forces or acceleration/deceleration forces.
In conclusion, therefore, the method proposed by Zenger et al. in U.S. Pat. No. 5,251,424 in theory can counteract the negative vacuum pressure created by a cooling hot filled product with a positive pressure from liquid nitrogen gas forming an expanding gas. However in practice there are a number of inaccuracies inherent in the dosing of liquid nitrogen as well as the precise handling and temperature of the product and bottle during the processing. The combination of peak internal pressure and minimum package resistance to internal pressure caused by elevated temperatures results in deformation of the plastic packaging to an unacceptable degree. Lack of predictability, and waste of materials and product have resulted in failure of the Zenger method in commercial applications.
For example, the inventors conducted a test of the Zenger et al. method with the following results. A 600-ml. heat set PET bottle with a petaloid base was filled with water at 185xc2x0 F. One gram of dried ice was quickly deposited within the hot water and the bottle was capped within several seconds. The hot filled bottle was left at rest on a horizontal surface with no lateral supports or restraints. The bottle base experienced severe roll out within several seconds of capping, presumably as a result of the combination of increased internal pressure and decreased strength of the PET bottle due to elevated temperatures. The deformed bottle was then quenched in cold water, however the deformed base remained rolled out after quenching and cooling. When the bottle was opened, there was no residual internal pressure remaining in the bottle. The lateral sides of the body of the bottle had triangulated indicating that the product on cooling had decreased in volume and created an internal vacuum which collapsed the sides of the bottle into a triangular shape.
It is an object of the invention, to provide a method by which plastic containers can be utilized in packaging products which require exposure to the combination of elevated temperatures and internal pressure variations during processing.
It is a further object of the invention to avoid the disadvantages of the prior art in utilizing plastic containers including avoidance of heat set plastic containers which are relatively expensive, avoidance of relatively heavy walled plastic containers which are also expensive compared to conventional bottles, and avoidance of use of relatively expensive high strength plastics.
It is a further object of the invention to utilize conventional PET plastic containers without vacuum collapsible panels or other special features in a hot-fill, pasteurized or retort process thereby using the conventional plastic containers during product filling and packaging when exposed to both elevated temperatures and internal pressure variations without experiencing deformation or structural failure.
Further objects of the invention will be apparent from review of the disclosure and description of the invention below.
A method of supporting a plastic container exposed to elevated temperatures during product filling and packaging to eliminate heat and internal pressure induced distortion, internal vacuum distortion, or structural failure. Plastic containers usually have a body with exterior configuration and a sealable open mouth. The invention provides a method of hot filling, pasteurising and retort processing of conventional plastic containers during product filling where at least a portion of the container is confined in a support casing having an interior cavity mating the exterior configuration of the container body portion. The product is then introduced into the container, and the container mouth is sealed. A positive pressure is induced within the sealed container while the container is exposed to a product processing temperature. An unsupported PET container exposed to high heat and internal pressure variations would distort or fail. However, the external mating support casing together with positive internal pressure restrains the PET container during this period of high stress and low resistance thereby preventing distortion or failure. Afterwards, by cooling the container to a casing release temperature below the product processing temperature and then releasing the container from the casing eliminates container distortion problems.
A significant advantage of the invention is that it enables use of the conventional non-heat set bottles with no special moulds and no special material to create containers for use in hot filling, pasteurisation or retort processing.
A primary benefit is in a reduction in material costs. Conventional (non-heat set) soft drink type bottles weigh substantially less than relatively heavy heat set bottles. Adopting the method of the invention, such conventional soft drink bottles can be used for hot filling, pasteurisation or retort processing thus replacing heat set bottles.
Increased bottle moulding productivity also results since mould machines can output more lightweight soft drink type bottles per mould per hour than heat set bottles. Heat setting requires longer mould residence time and thus decreases bottle moulding cycle output.
The invention also increases design freedom providing more options to the designer than with typical heavy weight heat set bottles. Use of conventional bottles that can be filled using the hot filling, pasteurisation or retort process, as well as used to package soft drinks or other products by conventional methods has significant beneficial results on decreased material costs, increased productivity, increased design freedom, reduced inventory, reduced storage requirements, improved scheduling and reduced mould maintenance costs.
The soft drink and packaged drink markets are extremely seasonal with high demand during summer periods, which require stock piling in advance of the peak period to meet the peak demand. Storage of finished bottles in order to meet the customer""s orders during peak periods is a significant expense and involves considerable risk on the part of the bottle manufacturer. Conventional hot-filled bottles require specialized collapsible vacuum panels and individual moulds. Therefore, conventionally the stock piling of different bottle designs are required in order to meet peak demand. By utilising the same bottles for soft drinks and hot fill applications, significant savings in inventory expense, storage as well as mould design and maintenance are highly advantageous results of the invention.
In manufacturing of containers, the elimination of specialized designs is a significant advantage. Identical moulds can be used thereby reducing tooling, down time and change over costs. Use of identical moulds avoids the need for maintenance of specialized moulds for different products.
Minor changes to filling machine operations are required in order to accommodate the external bottle restraints and addition of liquid nitrogen or dry ice. It is expected that in many cases the lateral restraints can simply take the form of a light weight plastic armour, sleeve or tube casing that is wrapped around the bottles as they are conveyed with conventional equipment that contacts only the bottle finish or spout area. Bottles and other containers are generally conveyed and handled using only the neck or rim portion of a container. In blow moulded bottles especially, the finish mouth and neck portion are significantly thicker material than the blow moulded body portion due to the blow moulded manufacturing method. The thicker areas are able to resist the internal pressure and elevated temperatures without external support or restraint. The thinner base area and sidewalls are supported or restrained without interfering with conventional handling equipment.
As a result the invention permits the market expansion of plastic containers into the areas that are conventionally served by glass and metal containers such as retort processing of foods. Increased design freedom, lower manufacturing costs, lower weight and shipping costs and simplification of inventory for manufacturing and storage result.
Using conventional methods such as proposed in U.S. Pat. No. 5,525,124 to Zenger et al., the PET container must be designed to be free standing and resist maximum internal pressure at the same time as the container is subjected to maximum heat. When exposed to heat the container has significantly reduced capacity to resist the internal pressure and deformation exactly when the maximum life cycle capacity is required.
Using conventional methods, the bottle must be over designed to avoid deformation and collapse during processing since maximum lifetime stress on the bottle results from the coincidence of maximum internal pressure and maximum exposure to heat (i.e. minimum container strength).
The invention on the other hand provides means to support and reinforce the container during the maximum stress and maximum heat period during processing. Once the container has survived the peak internal pressure and maximum heat exposure during processing, the container is not required to exhibit the same maximum resistance during storage and transport. The invention provides means to temporarily reinforce the bottle during maximum internal pressure and heat exposure. This method allows use of conventional bottles or containers, that have performed satisfactorily for soft drinks and cold filled products, to be used for hot filled, pasteurised and retort processed products.
Utilising the method of the invention successfully avoids the permanent deformation and distortion which results from use of conventional bottles for these heat inducing processes in the prior art.